“Violence and injury enclose in their net all that do such things, and generally return upon him who began.”
Book V, lines 1152–1153 (tr. Rouse)
De Rerum Natura (On the Nature of Things)
Original
Circumretit enim vis atque iniuria quemque, atque, unde exortast, at eum plerumque revertit.
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Lucretius 45
Roman poet and philosopher -94–-55 BCRelated quotes

Semayne's Case, 77 Eng. Rep. 194, 195; 5 Co. Rep. 91, 195 (K.B. 1604).

“The best revenge is to be unlike him who performed the injury.”
VI, 6
Variant: The best revenge is not to be like your enemy.
Source: Meditations (c. 121–180 AD), Book VI

This passage contains some phrases King later used in "Where Do We Go From Here?" (1967) which has a section below.
1950s, Loving Your Enemies (Christmas 1957)
Variant: Returning hate for hate multiplies hate, adding deeper darkness to a night already devoid of stars. Darkness cannot drive out darkness; only light can do that. Hate cannot drive out hate, only love can do that.
Source: Mentioned in "Out of Osama's Death, a Fake Quotation Is Born" by Megan McArdle, The Atlantic (May 2011) http://www.theatlantic.com/national/archive/2011/05/out-of-osamas-death-a-fake-quotation-is-born/238220/, and widely distributed on twitter http://twitter.com/#!/jmadly/status/65314784136011776 as a quote of King, after the death of Osama bin Laden, the first sentence is one written by Jessica Dovey http://i.imgur.com/cqtjw.jpg on her Facebook page, which became improperly combined by others with genuine statements of King, whom she quoted, and which occur in Strength to Love (1963), Ch. 5 : Loving your enemies, and in Where Do We Go from Here : Chaos or Community? (1967), p. 62.
For the full story see "Anatomy of a Fake Quotation" by Megan McArdle, The Atlantic (May 3, 2011) http://www.theatlantic.com/national/archive/2011/05/anatomy-of-a-fake-quotation/238257/ and for the Facebook version of the quote see Did Martin Luther King, Jr. say that “I will not rejoice in the death of one, not even an enemy”? at skeptics.stackexchange.com http://skeptics.stackexchange.com.
Context: Returning hate for hate multiplies hate, adding deeper darkness to a night already devoid of stars. Darkness cannot drive out darkness; only light can do that. Hate cannot drive out hate; only love can do that. Hate multiplies hate, violence multiplies violence, and toughness multiplies toughness in a descending spiral of destruction.
Context: Let us move now from the practical how to the theoretical why: Why should we love our enemies? The first reason is fairly obvious. Returning hate for hate multiplies hate, adding deeper darkness to a night already devoid of stars. Darkness cannot drive out darkness; only light can do that. Hate cannot drive out hate; only love can do that. Hate multiplies hate, violence multiplies violence, and toughness multiplies toughness in a descending spiral of destruction. So when Jesus says "love your enemies," he is setting forth a profound and ultimately inescapable admonition. Have we not come to such an impasse in the modern world that we must love our enemies-or else? The chain reaction of evil-Hate begetting hate, wars producing more wars-must be broken, or we shall be plunged into the dark abyss of annihilation.
Context: I mourn the loss of thousands of precious lives, but I will not rejoice in the death of one, not even an enemy. Returning hate for hate multiplies hate, adding deeper darkness to a night already devoid of stars. Darkness cannot drive out darkness: only light can do that. Hate cannot drive out hate: only love can do that.

“It is not violence that best overcomes hate — nor vengeance that most certainly heals injury.”
Helen Burns to Jane (Ch. 6)
Jane Eyre (1847)
Context: It is not violence that best overcomes hate — nor vengeance that most certainly heals injury. … Read the New Testament, and observe what Christ says, and how he acts — make his word your rule, and his conduct your example. … Love your enemies; bless them that curse you; do good to them that hate you and despitefully use you.

Reported in Benjamin H. Hill, Jr., Senator Benjamin H. Hill of Georgia; His Life, Speeches and Writings (1893), epigraph, p. 594. From "Notes on the Situation", a series of articles appearing in the Chronicle and Sentinel, Atlanta, Georgia.

Source: Bury My Heart at Wounded Knee (1970), p. 94

A covering note sent with a manuscript submission, which was supposedly returned with the answer, "Put this with your other irons." The same story had much earlier been told about Dr. Johnson and Mrs. Piozzi in Kate Sanborn Home Pictures of English Poets (New York: Appleton, 1869) p. 215.
Misattributed

Against the Galilaeans (c. 362)
Context: If the immediate creator of the universe be he who is proclaimed by Moses, then we hold nobler beliefs concerning him, inasmuch as we consider him to be the master of all things in general, but that there are besides national gods who are subordinate to him and are like viceroys of a king, each administering separately his own province; and, moreover, we do not make him the sectional rival of the gods whose station is subordinate to his. But if Moses first pays honour to a sectional god, and then makes the lordship of the whole universe contrast with his power, then it is better to believe as we do, and to recognise the God of the All, though not without apprehending also the God of Moses; this is better, I say, than to honour one who has been assigned the lordship over a very small portion, instead of the creator of all things.